1. Field of the Invention.
The present invention relates to a kit for and a method of reducing the sound for model engines in a low cost manner without complete replacement of existing mufflers or manifold headers.
2. Description of the Prior Art.
Various muffler adaptations have been made for model engines such as model airplane engines in particular, which are quite noisy, and which now have been subjected to stringent recommended standards of noise reduction. Many mufflers that attempt to keep back pressure on the engine low and also adequately reduce sound have been advanced, but these have been very expensive, and still have problems meeting the recommended noise limits.
Most of the small displacement engines use a two-piece "muffler" as illustrated in FIG. 1 of the present drawings, which is exemplary to the prior art. As shown in FIG. 1, a model engine of small displacement indicated generally at 10 in dotted lines has an output flange at 11, on which a manifold-muffler assembly 12 is mounted. The muffler has a mounting flange 13 that bolts onto the engine, and an internal passageway 14 opening to the exhaust port of the engine. A single expansion chamber is defined as indicated at 15 on the interior of the muffler 16 which includes a front half 17 and a rear half 18 joined along a threaded interconnection 19. A stinger pipe 20 is provided at the rear, and the single expansion chamber 15 will reduce the volume of sound, and at the same time, some back pressure is also produced for improved regulation of engine power. This can be a tuned type exhaust if desired.
A typical measured level of engine sound without any muffler will be in the range of 104 to 112 decibels (Db) at a distance of nine feet under a set of controlled test conditions. The addition of a factory stock muffler such as that shown in FIG. 1 reduces the sound about 80%, or in the range of 90 to 101 Db under the same set of controlled test conditions. Continuous sound over 90 Db is still painful and damaging to the human ear. It should be noted that each 3 Db amounts to a 50% sound reduction. In order to be "legal" or to meet guidelines, most engines must be reduced by at least 3 to 11 Db from factory muffler sound levels for them to operate satisfactorily.
By adding on standard factory mufflers and the like, back pressure is increased, and in addition the cost is substantially increased.